Tacoma Narrows Bridge Collapse

Jackie MoorePd. 6

What Happened?

The Tacoma Narrows Bridge was the third largest suspension span bridge in the world. The designer of the bridge was Leon Moisseiff, was one of the world's foremost bridge engineers. His intent was to produce a very slender deck span arching between two tall towers. The bridge was nicknames "Galloping Gertie" by people who experienced its strange behavior. Men who worked on the construction of the bridge often complained of seasickness because of the undulations of bridge that pitched and rolled the deck. Even after the opening of bridge, on July 1st, 1940, people were challenged to cross the bridge during light winds and complained of seasickness. Although Moisseiff had a top-ranked engineering reputation, State and Toll bridge Authority engineers were more than a little nervous about the behavior of the bridge. Engineers attempted multiple methods to try to stabilize, or dampen, the up-and-down waves of the deck. On November 7th, 1940 at 11:00 the bridge finally collapsed. Witness, Kenneth Arkin, reported the bridge's velocity to be 38-40 milers per hour while the deck rose and fell 3 feet 38 times in one minute at 10:00. Arkin halted traffic and watched the bridge begin to sway from side to side and twisting. By 10:30 the amplitude of the undulations was 25 feet deep and the suspender ropes began to tear. When the stiffening girder fell 190 feet it splashed a plume of water 100 feet into the air. Within a half hour, the rest of the deck fell section by section.

Facts and Figures

Loss of life: There was only one casualty of the collapse which was a dog named Tubby

Injuries: None

Financial Losses: -The Washington state filed an insurance claim for $5,200,000. The state and 22 insurance companies agreed on a settlement of $4 million.

What was to blame?

The engineers had not properly considered the aerodynamic forces that were in play at the location during a period of strong winds. At the time of construction, such forces were not commonly taken into consideration by engineers and designers.

In March 1941 the Carmody Board announced its findings. Three key points stood out: (1) The principal cause of the Narrows Bridge's failure was its "flexibility;" (2) the solid plate girder and deck acted like an airfoil, creating "drag" and "lift;" and (3) aerodynamic forces were little understood and engineers needed to test all suspension bridge designs thoroughly using models in a wind tunnel.

Physics behind the failure

The Tacoma Narrows bridge collapsed due to wind induced resonance. Resonance is a process in which an object's, in this case a bridge's, natural vibrating frequency is amplified by an identical frequency. In this case the identical frequency was caused by strong wind gusts blowing across the bridge, creating regions of high and low pressure above and below the bridge. This produced the change from vertical waves to destructive twisting, torsional motion, ending in a collapse. the wind was forced either above or below the bridge, causing the bridge to be moved up or down. This tensed or relaxed the supporting cables, which acted much like rubber bands, and increased the waves in the bridge. These waves were so intense that a person driving across the bridge often lost sight of the car ahead as it dropped into a trough, low point, of the wave.

Impact

The study of wind behavior grew into an entire engineering discipline called aerodynamics.

No bridge, building or other exposed structure was designed without testing a model in a wind tunnel.

Dozens of papers are published each year about these subjects

A new bridge was built in 1950 and tested in wind tunnels

The Tacoma Narrows Bridge collapse remains one of the most spectacular failure in the history of engineering.